Method and apparatus for signal processing using reference signals

ABSTRACT

The invention provides methods and apparatus for substituting a reference spread spectrum signal for a pilot tone or providing a reference spread spectrum signal where no pilot tone has been used. The invention has particular relevance when a large number of input channels are required for a broad band linear amplifier and when an omni-directional antenna is required for a repeater which may be prone to instability.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for substitutinganother type of signal for a pilot signal where the pilot signal wouldprovide a reference for level and/or frequency and/or phase, forexample. The invention also, of course, relates to applications where areference signal is required but no pilot signal has previously beenused or contemplated.

BACKGROUND OF THE INVENTION

The disadvantages of pilot signals include additional loading for anycircuit which uses the pilot signal, the provision of a slot in thefrequency spectrum for a pilot signal where it does not interfere withother signals, and where the pilot signal is used to characterizechanges in a transmission path, the characterization is strictlyspeaking only applicable to the frequency region occupied by the pilotsignal.

The present invention is particularly useful in the removal ofdistortion in broadband linear amplifiers such as are described in U.S.Pat. Nos. 5,334,946 and 5,157,345. Another method of reducing distortionin such amplifiers is described in U.S. Pat. No. 4,580,105 assigned toA.T & T (inventor Myer). The correction method of this U.S. Pat. No.4,580,105 suffers from the above mentioned disadvantages and a furtherdisadvantage in that the pilot signal restricts channel usage within theamplifier where a multi-channel input is applied to the amplifier.Previously mentioned U.S. Pat. No. 5,334,946 and 5,157,345 overcamethese problems by avoiding the use of a pilot signal but a furtherproblem exists in the particular situation where it is required tocombine a large number of input channels for application to thebroadband linear amplifier. Such an amplifier is usually used as thefinal power amplifier of a radio transmitter and for this reason it mustbe near a transmission antenna. The antenna is usually situated on thetop of a building where rentals are high. A large number of inputcables, one for each input channel, for example 30 to 100, have to berun to the top floor of this building. In addition some circuits have tobe duplicated for each channel near the amplifier and high rental spaceis required for this purpose.

Another important application of the invention is in radio repeaters,especially for cellular radio, where a repeater for a "hole" incoverage, for example in a tunnel, receives signals at a low level andtransmits at a high level on the same frequency. Instability is likelyin such repeaters but can usually be avoided by using directionalantennas and positioning transmit and receive antennas as far apart aspossible. However in many situations one omnidirectional antenna isrequired and suitable spacing is inconvenient. A similar applicationwhich uses a derivative of the technique is in frequency translatingrepeaters. An advantage then gained is the elimination of the complexfiltering otherwise required in these systems.

In code-division multiple access (CDMA) systems each channel employs aspread spectrum signal and the channel signals are added beforetransmission. U.S. Pat. No. 4,962,507 describes a system in which apilot signal for transmitting a timing reference is also transmitted inspread spectrum form.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided amethod of processing signals comprising the steps of

combining a spread spectrum signal with an input signal to be processed,

processing the combined signals to provide an output signal,

deriving an intermediate signal dependent on the processed combinedsignals and adjusting the phase and/or amplitude of the intermediatesignal in response to control signals,

combining the adjusted intermediate signal with the input or the outputsignal of the processing to improve processing, and

deriving the control signals from the output signal.

Methods according to the first aspect of the invention, andcorresponding apparatus, may be used where the signals to be processedare spread spectrum signals.

According to a second aspect of the present invention there is provideda method of processing signals comprising the steps of combining areference signal, having a predetermined characteristic, with the inputsignals which are not spread spectrum modulated,

processing the combined input-and reference signals to provide processedsignals, and

extracting a signal representative of the characteristics from theprocessed signals to control, at least partially, the processing and/orfurther processing of the processed signals,

wherein the reference signal is a spread spectrum signal.

The second aspect of the invention is based on the realization that aspread spectrum reference signal can be used in a system which does nototherwise employ spread spectrum signals.

The advantages of the invention include not having to provide a portionof the spectrum for a pilot signal so that for example in multichannelusage more of the spectrum is available for channels. Since the spreadspectrum signal is a noise-like signal little additional load is imposedat any particular frequency and any components remaining in outputsignals appear as noise.

Thus the invention can be used in many applications to replace aconventional pilot signal. Most advantage is gained with wide bandwidthsystems and for narrow band systems a conventional pilot signal isusually better.

Where the reference signal is used to characterize a circuit ortransmission link, the characterization is broadband.

A number of respective spread spectrum reference signals can be usedsimultaneously if orthogonal random sequences are used to generate thespread spectrum signals, since each reference signal can be compared, orits non-random generating signal can be recovered, using its particularreference sequence. Thus multiple reference signals can be used in thesame circuit for different purposes, or for similar purposes indifferent parts of the same circuit.

Other advantages include the ability to work with any modulation scheme.Most control techniques can be easily modified for use with spreadspectrum reference signals since conversion to this form and comparison,or recovery of a non-random signal, employs simple known methods andcircuits.

In the radio repeater application mentioned above, the spread spectrumsignal can be used, as is described in more detail below, to identify aportion of a transmitted signal which has been received by therepeater's receive antenna. This portion is then subtracted from theinput signal to the repeater's amplifier after phase and amplitudeadjustment in much the same way as is the broad band amplifierapplication to provide an interference free signal for transmission athigh power to avoid instability.

Extracting the said characteristic may include using a spread spectrumreference signal or a replica thereof for comparison with the spreadspectrum signal forming part of the processed signals, or recovering anon-random signal used in generating the spread spectrum signal from theprocessed signals. Recovery may be using the same random sequence as forconversion; for example by using the same or a similar random signalgenerator for recovery as for conversion.

Since the characteristic can be used either in controlling processingafter extraction of the characteristic or before extraction, control canbe seen to be either feedforward or feedback.

The characteristic may or may not be modified when the combined inputand reference signals are processed.

In the broadband amplifier application mentioned above, it is the degreeto which the reference signal exists in the amplifier output which isimportant, that is its level, since in this application the referencesignal can be regarded as a form of specifically introduced distortionsignal.

Thus in some forms of processing the characteristic may be modifiedbetween combination and use, and it may be that the degree or way inwhich modification has occured is the function of the characteristicwhich is needed for processing.

In another example, the level of the reference signal may be thecharacteristic, for instance when processing includes transmission bysome means from one location to another and variable attenuationexperienced by the input and reference, signal is to be compensated whenprocessing is carried out.

In some other forms of processing the characteristic of the referencesignal may be one which is required to be as far as possible unchangedbetween combination with the input signals and its use in the control ofprocessing. For example, the frequency of the above mentioned non-randomsignal may be the characteristic, for instance when processing comprisesdemodulation using a signal derived from the reference signals.

Thus the characteristic may be frequency or signal level, but otherpossibilities include phase, or a combination of characteristics.

The spread spectrum signal may be of any type, for example, onegenerated by changing frequency of a signal generator according to arandom sequence or by combining a series of pulses representing "ones"and "zeros" with a tone. Other systems which may be used include timehopping in which signal bursts are randomly sequenced in time and chirpsystems in which each signal burst has a narrow frequency bandwidthwhich rapidly sweeps the entire bandwidth.

According to a third aspect of the invention there is provided abroadband linear amplifier comprising

amplifier means,

first difference means for deriving an error signal, representative ofdistortion introduced by the amplifier means, from signalsrepresentative of input signals applied to the amplifier means.

second difference means for subtracting a signal representative of theerror signal from the amplifier means output signals to provide lowdistortion output signals, and

adjustment means responsive to control signals for adjusting therelative phase and amplitude of signals applied to at least one of thedifference means,

wherein means are provided for generating a spread spectrum referencesignal and combining the reference signal with the input signals afterthe signals representative of the input signals are derived from thefirst difference means, and

control means are also provided for generating the control signals fromboth the spread spectrum signal as it appears in the low distortionsignals and a signal representative of the spread spectrum signal usedfor combination with the input signals or the error signal.

According to a fourth aspect of the invention there is provided arepeater comprising

an amplifier coupled between receive and transmit points,

means for deriving a cancellation signal representative of the amplifiedoutput signal,

means for adjusting the amplitude and phase of the cancellation signalin response to control signals,

difference means for subtracting the cancellation signal from signalsreceived at the receive point to provide input signals for the amplifierhaving reduced interference between signals to be amplified by therepeater and signals transmitted at the transmit point,

means for combining the input signals with a spread spectrum referencesignal, and

means for deriving the control signals from the reference signal and thesaid input signals.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the invention will now be described withreference to the accompanying drawings, in which:

FIG. 1 is a block diagram of a broadband linear amplifier according tothe invention,

FIGS. 2 and 3 are block diagrams of further broadband linear amplifiersaccording to the invention, each employing a digital signal processor toact as a feedback control network,

FIG. 4 is a block diagram of a radio repeater according to theinvention, and

FIG. 5 is a block diagram of a circuit which may be used in a radiorepeater where time delayed interference terms may occur.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is similar to FIG. 1 of the above mentioned U.S. Pat. Nos.5,334,946 and 5,157,345 except that the feedback network 20 developscontrol signals for the phase adjustment component 13 and the gainadjustment 14 partly by means of the spread spectrum technique of thepresent invention. However, FIG. 1 will be described as a whole leavingsome details to be obtained from the earlier application.

An input signal is applied at an input terminal 1 and is divided by asplitter 2 between two paths: a main path 3 to a main amplifier 4 and asubsidiary path 5 to phase and gain adjustment components 6 and 7. Theoutput signal from the main amplifier 4 includes distortion products inthe form of intermodulation but in addition a spread spectrum signal isintroduced at a combiner 25. An RF signal generator 26 supplies an RFcarrier at the center of the frequency band of the input signals to amixer 27 which also receives squarewave, or filtered squarewave, signalsfrom a maximum length pseudo-random sequence generator 28 at basebandand having a bandwidth equal to or less than that of the input signals.In this known way a direct sequence spread spectrum signal (see"Principles of Communication Systems" (2nd Edition), H. Taub and D. L.Schilling, McGraw Hill 1986, Chapter 17, pages 720 to 749) is applied byway of the combiner 25 to the input of the main amplifier 4. Thus theoutput from the amplifier 4 includes not only the distortion productsbut also the spread spectrum signal spread across the whole band of theinput signals by means of the random noise-like signal from thegenerator 28 which is mixed with the RF signal from the generator 26. Ofcourse other methods of introducing a spread spectrum signal across thewhole or part of the band of the input signal may be used to inject aspread spectrum signal into the input of the main amplifier 4.

A portion of the main amplifier output is obtained from a directionalcoupler 9 and fed as a first input to a combiner 11. A second input forthe combiner 11 from the input terminal 1 is arranged to be in antiphaseto the portion of the power amplifier output (thus forming a subtractor)by select,on of a time delay element 8, and a correct adjustment of thephase shift component 6. For optimum cancellation of the input signal toprovide an error signal at the output of the combiner 11, the amplitudelevels must also be adjusted and this is arranged by correct adjustmentof the variable gain component 7. The signal obtained from the output ofthe subtractor 11, in theory, contains only the distortion products andthe spread spectrum signal, and thus forms an error signal.

The error signal is used to cancel the distortion products and thespread spectrum signal present in the output of the main amplifier 4.The main amplifier signal at the output of the directional coupler 9 isdelayed by a time delay element 16 and fed to one input of a directionalcoupler 17 acting as a subtractor. The other input of the directionalcoupler 17 is obtained by processing the error signal derived previouslyfrom the combiner 11 (acting as a subtractor) using a time delay element12, phase and gain adjustment components 13 and 14, and an erroramplifier 15. The variable gain and phase shift components 13 and 14 areadjusted for maximum cancellation of the unwanted distortion productspresent in the output signal of the coupler 17 and also to allow forphase and amplitude errors in the amplifier 15. The presence of thespread spectrum signal allows adjustment of the components 13 and 14 tobe optimized for the whole frequency band and to operate when distortionproducts are at a very low level.

Adjustment of the phase and gain components 6 and 7 is by means of afeedback network 18 which is as described in the above mentioned patentsand which derives input signals from couplers 10 and 19. Correction ofphase and gain of the error signal generated at the output of thecombiner 11 is under the control of a feedback network 20 which appliescontrol signals to the phase and gain adjustment components 13 and 14.The network 20 is also as described in the previous specification, thatis it is the same as the network 18 but one of the signals applied tothe input of this network comes from a different point from that of theprevious application. The signal which is the same comes from the samepoint, and is from a directional coupler 21 while the other signal comesfrom the mixer 27 by way of a time delay element 31: that is, it is thespread spectrum signal injected at the input of the amplifier 4. Thusthe feedback network 20 is able to compare any remnant of the spreadspectrum signal in the coupler 21 with the original injected spreadspectrum signal.

In operation the error signal at the output of the combiner 11 whichcontains both distortion components and the spread spectrum signalpasses through the phase and gain adjusters 13 and 14 to the subtractioncircuit 17 where the phase and gain of the distortion components and thespread spectrum signal are such that these components and thecorresponding spread spectrum signal from the amplifier 4 are cancelled.Any residual distortion components or spread spectrum signal componentsare fed back from the directional coupler 21 to the feedback network 20.As a result of the two input signals it receives this network adjuststhe phase component 13 and the gain component 14 such that cancellationin the circuit 17 is optimum. Since the spread spectrum signal is spreadover the whole bandwith the adjustment of the components 13 and 14 isoptimum for the whole bandwidth.

In the above mentioned previous specification several forms of thefeedback network 20 were described and in one a digital signal processor(DSP) was used. As explained in the previous specification it ispreferable that a DSP operates at audio frequencies and for this reasonthe circuit of FIG. 2 may be used.

FIG. 2 is similar to FIG. 1 except that the feedback network 20 is inthe form of a DSP (not shown) and circuits are provided to give inputsignals for the network at audio frequencies. A generator 32 provides asignal at a frequency offset from that of the generator 26 by an amountequal to the input frequency required by the DSP to one input of each ofmixers 29 and 33. Thus the generators 26 and 32 may have output signalsat 900 MHz and 900.00125 MHz for example. The mixer 29 then has anoutput at 1.25 KHz and, assuming that the M-sequence generator 28 has anoutput signal band from 0 to 30 MHz, the output of the mixer 33 has anoutput signal band from 900.00125 to 930.00125 MHz. When this latterband is mixed with the signal from the coupler 21 in a mixer 30, theoutput is an error signal at 1.25 KHz. The DSP thus receives inputsignals at suitable frequencies and may be programmed in the same waysas described in U.S. Pat. Nos. 5,334,946 and 5,157,345.

The generators 26 and 32 may include integrated circuit signalgenerators derived from a 10 MHz reference with the appropriate outputsignals derived therefrom either internally or externally. Thegenerators should be frequency locked as indicated by a connection 22.

Where a number of input channels are to be combined at one location andthen applied as an input signal to the amplifier 4 and the distortionminimizing circuits of FIG. 1 at a remote location, then the inputsignals are added, in the same way as shown later in FIG. 3, beforeapplication to the input terminal 1. A suitable connection joins theadditional circuit used for this purpose and the splitter 2. Theadvantage obtained is that the numerous splitters 23 of FIG. 2 of theabove mentioned U.S. Pat. No. 5,157,345 and the phase and gainadjustment components 24 and 25 of that figure which would have to be atthe same location as the amplifier 4 are no longer required and a singleconnection replaces the inputs to the combiner 28 of that figure. Theremay be thirty or more input channels so the circuits of FIGS. 1 and 2 ofthe present application provide a useful advantage especially wheresumming can be carried out at a relatively low rental location such asthe basement of a building and transmission using the amplifier 4 mustbe carried out at the top of a building at an expensive rental location.The numerous connections previously required between the two floors arenow replaced by a single connection.

The arrangement of FIG. 3 illustrates that the invention can be appliedto a broadband linear amplifier employing several loops for distortioncorrection and in which the adjustment of phase and gain is applied tothe input signals to the main amplifier 4. FIG. 3 has some similaritiesto FIG. 2 of the above mentioned application but it differs in oneimportant aspect: the two input channels illustrated, which may berepresentative of many more, are applied to an addition circuit 34before any phase or gain adjustment is carried out so the two inputsillustrated and the addition circuit 34 can be located at a remotelocation from the rest of the circuit. This is possible according to theinvention because of the spread spectrum correction technique used.

A spread spectrum signal is added to the output of the gain adjustmentcomponent 36 in an adder 51, but the components 35, 36 and 51 can beconnected between the circuit 34 and the amplifier 4 in any order.

In the multi-loop control system of FIG. 3 a first error signal isproduced at the output of the combiner 11 as in FIGS. 1 and 2 but inthis case two phase and gain adjustment components 35 and 36 at theoutput of the addition circuit 34 and in the input path of the amplifier4 are used instead of the elements 6 and 7 in the path of one of theinputs to the combiner 11. Since FIG. 3 is intended to be a generalfigure a feedback network 37 is shown which receives as one of its inputsignals the error signal from the output of the combiner 11 but itsother input signal can be taken from a variety of different positionssuch as the output of the amplifier 15. The network 37 can be used tocontrol either or both of the pairs of phase and gain adjustmentcomponents 13 and 14, and 35 and 36 but either pair can, in somecircumstances, be manually adjusted initially and left at the originaladjustment during operation of the circuit.

In order to improve the cancellation of distortion components a seconderror signal is derived by a combiner 38 which receives the outputsignal from the first loop by way of a directional coupler 40 andsubtracts the output of the addition circuit 34 received by way of thedelay element 41 and phase and gain adjustment components 42 and 43. Thesecond error signal produced by the combiner 38 is applied by way ofphase and gain adjustment components 44 and 45 and an amplifier 46 to adirectional coupler 47 where cancellation of residual distortion occurs.A final output signal from the circuit of FIG. 2 is taken from adirectional coupler 48 to form one input for a feedback network 50 whichprovides control signals for the adjustment of the phase and gaincomponents 44 and 45. Another feedback network can be used to providecontrol signals for the components 42 and 43, or these components can beadjusted manually when the circuit is set up but are not then normallyadjusted again.

The network 50 is similar to that shown in FIG. 5 of U.S. Pat. Nos.5,334,946 and 5,157,345, where a DSP 60 is used, but one of the inputsignals is from the random sequence generator 28, and the frequencies ofoscillators 65 and 66 are different. The random sequence generates aspread spectrum signal at the output of a mixer 63 for application to amixer 70 where the output spread spectrum signal from the coupler 48,translated in frequency by a mixer 64 forms another input signal. Theoutput of this mixer forms the error signal for the DSP 60. Twooscillators 65 and 66, with output frequencies f₁ and f₂ chosen to givesuitable low frequency inputs to the DSP 60 apply inputs, for frequencytranslation to the mixers 63 and 64, respectively. Frequencies f₁ and f₂may for example be 457.50125 MHz and 457.5 MHz, respectively (assumingthe frequency of the generator 26 is 900 MHz and the bandwidth of the Msequence is 30 MHz). The output of the mixer 63 is given by:

    457.50125-30/2=442.50125 MHz

and that of the mixer 64 is at 442.5 MHz after selection of lower sidebands by filters 67 and 68. The error signal at the output of the mixer70 is thus at 1.25 KHz, and this frequency together with an output froma mixer 72 at a reference frequency of 1.25 KHz is applied to the DSP60. Band pass filters 73 and 74 select sidebands as appropriate. Theprogram for the DSP is again as described in U.S. Pat. Nos. 5,334,946and 5,157,345.

Nearly all the spread spectrum signal will be removed by cancellation inthe directional coupler 17 since it is seen by the feedback network 37as an error signal at the output of the amplifier 4. This is apparentsince the input signal to the combiner 11 which derives the error signalfrom the addition circuit 34 does not contain the spread spectrumsignal. It is only the residue which appears in the circuit output inthe directional coupler 48 which provides an error signal for thecontroller 50 and ensures additional cancellation in the directionalcoupler 47.

The provision of a spread spectrum signal source for connection to oneof the feedback networks on a DSP is not always necessary. For example,the spread spectrum signal from the combiner 38 of FIG. 3 can be used asone input to a network, such as the network 20 or, with suitablefrequency translation, a DSP, while the other inputs can be taken fromthe coupler 48. Both inputs contain corresponding spread spectrumsignals, and control signals are derived in the same way as thoughoperating on distortion products, as described in U.S. Pat. Nos.5,334,946 and 5,157,345.

It is clear that the invention as applied for example to broad bandlinear amplifiers can be put into operation in many other ways withdifferent arrangements of control loops and feedback networks producingcontrol signals for phase and gain adjustment. In particular each loopmay have its own specific spread spectrum correction according to thetechnique of the present invention. For this purpose a number oforthogonal random sequences, one for each loop, are mixed with theoutput of the RF signal generator 26 in a plurality of mixers. Circuitssuch as the network 20, or the network 50 are then provided for eachcontrol loop and receive a signal representative of its particularrandom sequence as a control-input signal.

As has already been pointed out, there are many different ways in whichthe multiple control loops can be arranged.

A radio repeater in the form of an enhancer for cellular radio using theinvention is now described, but this embodiment of the invention couldalso be modified for a line repeater.

The repeater of FIG. 4 is used where it is required to receive a signalon a frequency, amplify it and re-transmit on the same frequency. Wheremultiple channels require amplification as in cellular radio systems anamplifier 80 is preferably a broadband linear near amplifier of the typedescribed above or in the above mentioned U.S. Pat. Nos. 5,334,946 and5,157,345.

Signals received at a receiving antenna 81 pass by way of couplers 82,84 and 83 to the input of the amplifier 80. The output of the amplifier80 is connected by way of a coupler 85, acting as a splitter, to atransmit antenna 86. Weak wanted signals are received by the antenna 81,linearly amplified and re-transmitted at the antenna 86 but imperfectisolation between the antennas results in some of the transmitted energybeing coupled back into the receive antenna as unwanted, high level,interference which may cause instability. In order to avoid this problema signal equal to the interference signal and in phase therewith issubtracted from the incoming signal by the coupler 82, acting as asplitter. That part of the signal which has to be subtracted isidentified by a spread spectrum reference signal injected into theamplifier input using the coupler 83, acting as a splitter.

The spread spectrum reference signal is generated by using a mixer 87 tomix the outputs of an M-sequence generator 88 and an RF signal generator89. A portion of the output of the amplifier 80 is removed using thesplitter 85 and passed through gain and adjustment components 90 and 91to form the subtraction signal at the coupler 82 acting as a combiner. Anetwork 93 which may either be of the analogue type described in theabove mentioned patent application or, with suitable frequencytranslation, a DSP receives one input signal from the splitter 84 andone in the form of the spread spectrum reference signal from the mixer87. Any remaining interference signal in the signal received from thereceive antenna after subtraction of a signal nominally equal to theinterference signal in the coupler 82, acting as a combiner, is appliedto the network 93 and compared with the spread spectrum referencesignal. As a result the network 93 provides independent control signalsto adjust the amplitude and phase components 90 and 91 giving thecorrect amplitude and phase for the signal subtracted in the combiner82.

For cellular applications a bidirectional repeater is required and thiscan be achieved by using two circuits of the type shown in FIG. 4(without the antennas 81 and 86), one for each direction oftransmission. The two antennas are then connected to the two circuits ofFIG. 4 by way of hybrid circuits in the known way.

In situations where scattered (that is time-delayed) interference termsoccur they can be resolved and cancelled in a way similar to thatdescribed in FIG. 4 but using the arrangement shown in FIG. 5. Suchinterference terms may arise, for example, from different paths from thetransmit antenna 86 to the receive antenna 81. The transmitter output asobtained from the spitter 85 is applied to a terminal 94 where it passesthrough two delay circuits 95 and 96 which correspond to twotime-delayed interference terms. Three adjustment circuits 97, 98 and99, containing phase and amplitude adjustment components, receivesignals from the terminal 94 and the delay circuits 95 and 96,respectively, and provide outputs which are summed in a summer 101before being applied as the error signal to the combiner 82. Amplitudeand phase adjustment control signals for the circuit are derived in asimilar way to that shown in FIG. 4 from an M-sequence generator used asa reference signal and applied at a terminal 102, and a signal from thesplitter 84 applied at a terminal 103 after down-conversion to baseband, for example 0-30 MHz. A network 104 derives the amplitude andphase control signals from the spread spectrum reference signal and thesignal from the terminal 103 directly but control signals for thecircuits 98 and 99 are derived by circuits 105 and 106 from delayedversions of the reference signal provided by a first, and a secondinverse Z transformation, respectively, in circuits 107 and 108. Thecircuits 107 and 108 each provide a one-bit delay for the digitalsignals applied at the terminal 102. Although the signals at theterminal 103 are not strictly digital they are sufficiently close atcellular radio frequencies for almost all correlators to function. Ifnecessary, however, these signals could be processed, for instance by aSchmidt trigger circuit, in order to improve waveshape.

The reference signal from the mixer 87 and the signal from the splitter84 may, in an alternative arrangement, be used without down-conversionto derive amplitude and phase adjustment control signals.

The arrangement of FIG. 5 can be regarded as effectively creating atapped delay-line filter whose characteristics are frequency dependent.The arrangement is akin to a Rake receiver for spread-spectrumtransmissions (see: G. L. Turin, "Introduction to spread-spectrumantimultipath techniques and their application to urban digital radio",Proceedings of the IEEE, Vol. 68, No. 3, March 1980, pp. 328-353; and S.A. Allpress, M. A. Beach, G. Martin and C. M. Simmons, "An investigationof RAKE receiver operation in an urban environment for various spreadingbandwidth allocations", "Proceedings of the 42nd IEEE VehicularTechnology Conference", Denver, Colo. May 1992). The lower half of FIG.5 (circuits 104 to 108) may be implemented digitally in an ASIC.Down-conversion (as mentioned above) is then necessary.

The invention can be put into practice in many other ways where it isrequired to remove or reduce unwanted signals by cancellation or where acombination of a processed signal or a signal to be processed with aderived signal of controlled amplitude and/or phase improves processing.

We claim:
 1. A method of processing signals comprising stepsof:combining a spread spectrum reference signal with an input signal tobe processed into a combined signal; processing said combined signal toprovide a processed combined signal; deriving an intermediate signaldependent on said processed combined signal and Said input signal;adjusting at least one of a phase and an amplitude of said intermediatesignal in response to control signals to form an adjusted intermediatesignal; combining said adjusted intermediate signal with at least one ofsaid combined signal and said processed combined signal to form aresultant signal; and deriving said control signals from said resultantsignal.
 2. A method of processing signals according to claim 1,wherein:said step of processing said combined signal comprisesamplification; said step of deriving said intermediate signal comprisesderiving an error signal representative of distortion introduced by saidamplification; and said step of combining said adjusted intermediatesignal comprises subtracting said error signal from at least one of saidcombined signal and said processed combined signal to provide saidresultant signal.
 3. A method of processing signals according to claim1, wherein:said step of processing said combined signal comprisesamplification; said input signal includes spurious signals; and saidstep of combining said adjusted intermediate signal signal comprisessubtracting said adjusted intermediate signal from said at least one ofsaid combined signal and said processed combined signal to reduce saidspurious signals included in said input signal.
 4. A method ofprocessing signals according to claim 2 or 3, wherein:said controlsignals are derived from a signal representative of said resultantsignal and a signal representative of said spread spectrum referencesignal.
 5. An apparatus for processing signals comprising:means forcombining a spread spectrum reference signal with an input signal to beprocessed into a combined signal; means for processing said combinedsignal to provide a processed combined signal; means for deriving anintermediate signal dependent on said processed combined signal and saidinput signal; means for adjusting at least one of a phase and anamplitude of said intermediate signal in response to control signals toform an adjusted intermediate signal; means for combining said adjustedintermediate signal with at least one of said combined signal and saidprocessed combined signal to form a resultant signal; and means forderiving said control signals from said resultant signal.
 6. A method ofprocessing signals comprising steps of:combining a spread spectrumreference signal having a predetermined characteristic with inputsignals which are not spread spectrum modulated to form a combinedsignal; processing said combined signal to provide a processed combinedsignal; further processing said processed combined signal; andextracting an extracted signal representative of said characteristic ofsaid spread spectrum reference signal from said processed combinedsignal and said input signal to control, at least partially, saidfurther processing of said processed combined signal.
 7. A method ofprocessing signals comprising steps of:combining a spread spectrumreference signal having a predetermined characteristic with inputsignals which are not spread spectrum modulated to form a combinedsignal; processing said combined signal to provide a processed combinedsignal, said step of processing said combined signal comprisingamplification; further processing said processed combined signal;extracting an extracted signal representative of said characteristic ofsaid spread spectrum reference signal from said processed combinedsignal to control, at least partially, said further processing of saidprocessed combined signal deriving an error signal representative ofdistortion introduced by said amplification; subtracting said errorsignal from at least one of said processed combined signal and saidfurther processed processed combined signal; and using said extractedsignal to control an amplitude and a phase of said processed combinedsignal relative to said error signal before said error signal issubtracted from said processed combined signal.
 8. A method ofprocessing signals according to claim 6 or 7, wherein said step ofextracting said signal representative of said characteristic of saidspread spectrum reference signal includes using both an amount of saidspread spectrum reference signal remaining in said processed combinedsignal and said spread spectrum reference signal to derive a signaldependent on said characteristic of said spread spectrum referencesignal.
 9. A method of processing signals according to claim 8, furthercomprising steps of:generating a pre-reference signal at a centerfrequency of a frequency band of said input signals; and combining saidpre-reference signal with a pseudo random repeatable signal to generatesaid spread spectrum reference signal.
 10. A method of processingsignals according to claim 9, wherein said step of extracting saidsignal representative of said characteristic of said spread spectrumreference signal includes steps of:recovering a recovered signalrepresentative of said pre-reference signal from said spread spectrumsignal in said processed combined signal; and using both said recoveredsignal and a signal representative of said pre-reference signal toderive a signal dependent on said characteristic of said spread spectrumreference signal.
 11. An apparatus for processing signalscomprising:means for combining a spread spectrum reference signal havinga predetermined characteristic with input signals which are not spreadspectrum modulated to form a combined signal; means for processing saidcombined signal to provide a processed combined signal; means forfurther processing said processed combined signal; and means forextracting an extracted signal representative of said characteristic ofsaid spread spectrum reference signal from said processed combinedsignal and said input signal to control, at least partially, saidfurther processing of said processed combined signal.
 12. A broadbandlinear amplifier circuit comprising:amplifier means having an input andan output; first difference means for deriving an error signalrepresentative of distortion introduced by said amplifier means fromsignals representative of input signals applied to said input of saidamplifier means, said first difference means deriving a differencebetween a first signal representative of said input signal applied tosaid amplifier means and a second signal representative of said outputsignals from said output of said amplifier means; second differencemeans for subtracting a signal representative of said error signal fromoutput signals representative of said output of said amplifier means toprovide a low distortion output signal; adjustment means responsive tocontrol signals for adjusting a phase and an amplitude of signalsapplied to said first difference means simultaneously with a phase andan amplitude of signals applied to said second difference means; meansfor generating a spread spectrum reference signal and for combining saidspread spectrum reference signal with at least one of said input signalsand said signals representative of said input signals; and control meansfor generating said control signals from both an amount of said spreadspectrum reference signal remaining in said low distortion output signaland a signal representative of one of said spread spectrum referencesignal and said error signal.
 13. A broadband linear amplifier circuitaccording to claim 12, wherein said amplifier means comprises:anamplifier; and means for correcting distortion introduced by saidamplifier by generating an error signal and subtracting said errorsignal from an output signal from said amplifier.
 14. A broadband linearamplifier circuit according to claim 13, wherein said means forgenerating said spread spectrum reference signal comprises:means forproviding a signal at a center frequency of a band of frequencies to beamplified; means for providing a series of pulses having a bandwidthequal to that of said band of frequencies, said pulses having asignificance according to a pseudo-random repeatable sequence; and meansfor combining said signal at said center frequency with said series ofpulses to provide said spread spectrum reference signal.
 15. A broadbandlinear amplifier circuit comprising:amplifier means having an input andan output; first difference means for deriving an error signalrepresentative of distortion introduced by said amplifier means fromsignals representative of input signals applied to said input of saidamplifier means; second difference means for subtracting a signalrepresentative of said error signal from output signals representativeof said output of said amplifier means to provide a low distortionoutput signal; adjustment means responsive to control signals foradjusting a relative phase and an amplitude of signals applied to atleast one of said first difference means and said second differencemeans; means for generating a spread spectrum reference signal and forcombining said spread spectrum reference signal with at least one ofsaid input signals and said signals representative of said inputsignals, said means for generating said spread spectrum reference signalcomprising:means for providing a signal at a center frequency of a bandof frequencies to be amplified, means for providing a series of pulseshaving a bandwidth equal to that of said band of frequencies, saidpulses having a significance according to a pseudo-random repeatablesequence, and means for combining said signal at said center frequencywith said series of pulses to provide said spread spectrum referencesignal; and control means for generating said control signals from bothan amount of said spread spectrum reference signal remaining in said lowdistortion output signal and a signal representative of one of saidspread spectrum reference signal and said error signal.
 16. A broadbandlinear amplifier circuit according to claim 15 or 14, wherein saidseries of pulses comprises pulses having a first amplitude representinga first significance and pulses having a second amplitude representing asecond significance.
 17. A broadband linear amplifier circuit accordingto claim 15 or 14, wherein said control means for generating saidcontrol signals comprises:a digital signal processor; means forsupplying an offset signal at a frequency which differs from said centerfrequency of said band of frequencies by an amount equal to a frequencyat which said digital signal processor is to operate; first, second andthird mixer means; means for supplying a signal representative of saidseries of pulses to said first mixer means together with said offsetsignal in deriving an input signal to said second mixer means, whichreceives said low distortion output signal and provides a first inputsignal to said digital signal processor; means for supplying a signal atsaid center frequency of said band of frequencies to said third mixermeans together with said offset signal in deriving a second input signalto said digital signal processor.
 18. A broadband linear amplifiercircuit according to claim 15 or 14, wherein said control means forgenerating said control signals comprises:a digital signal processor;first and second mixer means; means for supplying first and secondoffset signals at respective frequencies which differ by an amount equalto a frequency at which said digital signal processor is to operate, tosaid first and second mixer means, respectively; means for supplying asignal representative of said series of pulses to said first mixermeans; means for supplying said low distortion signals to said secondmixer means; third mixer means for receiving said first and secondoffset signals and for providing a first input to said digital signalprocessor; and fourth mixer means for receiving outputs of said firstmixer means and said second mixer means and for providing a second inputto said digital signal processor.
 19. A broadband linear amplifiercircuit comprising:amplifier means having an input and an output; firstdifference means for deriving an error signal representative ofdistortion introduced by said amplifier means from signalsrepresentative of input signals applied to said input of said amplifiermeans, said first difference means deriving a difference between a firstsignal representative of an input signal applied to said amplifier meansand a second signal representative of an output signal from said outputof said amplifier means; second difference means for subtracting asignal representative of said error signal from output signalsrepresentative of said output of said amplifier means to provide a lowdistortion output signal; adjustment means responsive to control signalsfor adjusting a relative phase and an amplitude of signals applied to atleast one of said first difference means and said second differencemeans; means for generating a spread spectrum reference signal and forcombining said spread spectrum reference signal with at least one ofsaid input signals and said signals representative of said inputsignals, said means for generating said spread spectrum reference signalcomprising:means for providing a signal at a center frequency of a bandof frequencies to be amplified, means for providing a series of pulseshaving a bandwidth equal to that of said band of frequencies, saidpulses having a significance according to a pseudo-random repeatablesequence and means for combining said signal at said center frequencywith said series of pulses to provide said spread spectrum referencesignal; and control means for generating said control signals from bothan amount of said spread spectrum reference signal remaining in said lowdistortion output signal and a signal representative of one of saidspread spectrum reference signal and said error signal.
 20. A broadbandlinear amplifier circuit according to claim 16, wherein said controlmeans for generating said control signals comprises:a digital signalprocessor; means for supplying an offset signal at a frequency whichdiffers from said center frequency of said band of frequencies by anamount equal to a frequency at which said digital signal processor is tooperate; first, second and third mixer means; means for supplying asignal representative of said series of pulses to said first mixer meanstogether with said offset signal in deriving an input signal to saidsecond mixer means, which receives said low distortion output signal andprovides a first input signal to said digital signal processor; meansfor supplying a signal at said center frequency of said band offrequencies to said third mixer means together with said offset signalin deriving a second input signal to said digital signal processor. 21.A broadband linear amplifier circuit according to claim 16, wherein saidcontrol means for generating said control signals comprises:a digitalsignal processor; first and second mixer means; means for supplyingfirst and second offset signals at respective frequencies which differby an amount equal to a frequency at which said digital signal processoris to operate, to said first and second mixer means, respectively; meansfor supplying a signal representative of said series of pulses to saidfirst mixer means; means for supplying said low distortion signals tosaid second mixer means; third mixer means for receiving said first andsecond offset signals and for providing a first input to said digitalsignal processor; and fourth mixer means for receiving outputs of saidfirst mixer means and said second mixer means and for providing a secondinput to said digital signal processor.
 22. A method of broad bandlinear amplification comprising steps of:amplifying input signals toform amplified output signals; deriving an error signal representativeof distortion introduced by said step of amplifying input signals;subtracting said error signal from said amplified output signals toprovide low distortion signals; simultaneously adjusting a relativephase and an amplitude of at least one of signals used in deriving saiderror signal, said input signals, and said amplified output signals, inresponse to control signals; generating a spread spectrum referencesignal; combining said spread spectrum reference signal with said inputsignals; and generating said control signals from both an amount of saidspread spectrum reference signal remaining in said low distortionsignals and a signal representative of at least one of said spreadspectrum reference signal and said error signal.
 23. A radio repeatercomprising:an amplifier coupled between receive and transmit points;means for deriving a cancellation signal representative of an outputsignal from said amplifier; means for adjusting an amplitude and a phaseof said cancellation signal in response to control signals; differencemeans for subtracting said cancellation signal from signals received atsaid receive point; means for combining signals at said receive pointwith a spread spectrum reference signal; and means for deriving saidcontrol signals from at least one of said spread spectrum referencesignal and said signals at said transmit point.
 24. A radio repeateraccording to claim 23, comprising:means for deriving at least one timedelayed further cancellation signal representative of said output signalfrom said amplifier delayed in time; means for adjusting at least one ofan amplitude and a phase of said further cancellation signal in responseto further control signals; and means for deriving said further controlsignals from said spread spectrum reference signal, delayed in time, andsaid signals at said transmit point; wherein said difference means, inoperation, subtracts both said cancellation signals and said furthercancellation signals from signals received at said receive point toprovide said signals at said transmit point.
 25. A method of processinga signal comprising the steps of:combining a spread spectrum referencesignal with an input signal to generate a combined signal; processingsaid combined signal to provide a processed combined signal; extractingan intermediate signal dependent upon said processed combined signal;firstly deriving a first set of control signals; adjusting at least oneof an amplitude and a phase of said intermediate signal in response tosaid first set of control signals to provide an adjusted intermediatesignal; combining said adjusted intermediate signal with at least one ofsaid combined signal and said processed combined signal to provide aresultant signal; secondly deriving a second set of control signals;adjusting at least one of an amplitude and a phase of said resultantsignal in response to said second set of control signals to provide anadjusted resultant signal; wherein said first set of control signals andsaid second set of control signals enable said phase and said amplitudeof said intermediate signal to be adjusted concurrently with said phaseand said amplitude of said resultant signal.
 26. A method of processinga signal according to claim 25, wherein:said step of firstly derivingincludes deriving said first set of control signals from a signalrepresentative of said resultant signal; and said step of secondlyderiving includes deriving said second set of control signals from saidresultant signal and a signal representative of said spread spectrumreference signal.
 27. A broadband linear amplifier circuitcomprising:amplifier means having an input and an output; firstdifference means for deriving an error signal representative ofdistortion introduced by said amplifier means from signalsrepresentative of input signals applied to said input of said amplifiermeans; second difference means for subtracting a signal representativeof said error signal from output signals representative of said outputof said amplifier means to provide a low distortion output signal;adjustment means responsive to control signals for adjusting a relativephase and an amplitude of signals applied to at least one of said firstdifference means and said second difference means; means for addingsimulated distortion to at least one of said input signals and saidsignals representative of said input signals; and control means forgenerating said control signals from both an amount of said simulateddistortion remaining in said low distortion output signal and a signalrepresentative of one of said simulated distortion and said errorsignal.
 28. A broadband linear amplifier circuit according to claim 27,wherein said simulated distortion is added to said at least one of saidinput signals and said signals representative of said input signals byaltering at least one of an amplitude and a phase thereof.
 29. A methodof processing a signal comprising the steps of:combining a spreadspectrum reference signal with an input signal to generate a combinedsignal; processing said combined signal to provide a processed combinedsignal; extracting an intermediate signal dependent upon said processedcombined signal; firstly deriving a first set of control signals;adjusting at least one of an amplitude and a phase of said intermediatesignal in response to said first set of control signals to provide anadjusted intermediate signal; combining said adjusted intermediatesignal with at least one of said combined signal and said processedcombined signal to provide a resultant signal; secondly deriving asecond set of control signals; adjusting at least one of an amplitudeand a phase of at least one of said adjusted intermediate signal, saidcombined signal, and said processed combined signal in response to saidsecond set of control signals to provide an adjusted resultant signal;wherein said first set of control signals and said second set of controlsignals enable said phase and said amplitude of said intermediate signalto be adjusted concurrently with said phase and said amplitude of saidat least one of said adjusted intermediate signal, said combined signal,and said processed combined signal.
 30. A broadband linear amplifiercircuit comprising:amplifier means having an input and an output, saidamplifier means comprising:an amplifier, and means for correctingdistortion introduced by said amplifier by generating an error signaland subtracting said error signal from an output signal from saidamplifier; first difference means for deriving an error signalrepresentative of distortion introduced by said amplifier means fromsignals representative of input signals applied to said input of saidamplifier means, said first difference means deriving a differencebetween a first signal representative of said input signal applied tosaid amplifier means and a second signal representative of said outputsignals from said output of said amplifier means; second differencemeans for subtracting a signal representative of said error signal fromoutput signals representative of said output of said amplifier means toprovide a low distortion output signal; adjustment means responsive tocontrol signals for adjusting a relative phase and an amplitude ofsignals applied to at least one of said first difference means and saidsecond difference means; means for generating a spread spectrumreference signal and for combining said spread spectrum reference signalwith at least one of said input signals and said signals representativeof said input signals, said means for generating said spread spectrumreference signal comprising:means for providing a signal at a centerfrequency of a band of frequencies to be amplified, means for providinga series of pulses having a bandwidth equal to that of said band offrequencies, said pulses having a significance according to apseudo-random repeatable sequence, and means for combining said signalat said center frequency with said series of pulses to provide saidspread spectrum reference signal; control means for generating saidcontrol signals from both an amount of said spread spectrum referencesignal remaining in said low distortion output signal and a signalrepresentative of one of said spread spectrum reference signal and saiderror signal.